What was found
CLOSING THE CABIN DOOR WITH PASSENGERS BOARDED, CHECK DOORS AND CABIN DOOR OPEN AMBER CAS MESSAGES ILLUMINATED. COMPLIED WITH QRH PROCEDURE, WHITE CHECK DOORS MESSAGE ILLUMINATED. WE RESET THE CABIN DOOR AND ALL RELATED MESSAGES EXTINGUISHED. FOLLOWING NORMAL TAKEOFF AND CLIMBOUT THROUGH 10,000 FEET WE HEARD A LOUD POP FOLLOWED BY RED CABIN ALTITUDE CAS MESSAGE AS WELL AS CABIN DOOR OPEN AND CHECK DOORS AMBER CAS MESSAGES. WE DECLARED AN EMERGENCY, DESCENDED BELOW 10,000 FT AND RETURNED UNEVENTFULLY TO SDL. ADJUSTED & RIGGED THE VENT DOOR LINKAGE FOR AN EQUAL & FLUSH SEAL WHEN THE VENT DOOR IS CLOSED. CONTINUED WITH THE ADJUSTMENT/ RIGGING OF EXTERIOR DOOR HANDLE LINKAGE FOR PROPER FLUSH STOW OF THE EXTERIOR CABIN DOOR HANDLE. ACCOMPLISHED MULTIPLE OPEN / CLOSE CABIN DOOR CYCLES, WITH NO FAULTS. RESTORED ALL ACCESS OF THE MAIN CABIN DOOR. WORK DONE IAW AMM 52-10-00 & 52-10-43.